Forming of thick-layer, hybrid electronic printed circuits

ABSTRACT

Thick-layer hybrid electronic printed circuits are formed by printing predetermined circuit pattern onto an insulating substrate by deposition of predetermined ink pattern thereupon, advantageously by silk-screening or masking, and thence baking said ink circuit pattern, and repeating the deposition/baking steps as required, the subject forming process featuring use of an insulating ink comprising a non-conductive metallic oxide extender, desirably cuprous oxide, which ink is thus either potentially conductive or potentially resistive, and the development of such conductivity or resistivity, after baking, by treating the ink pattern with a reducing agent, desirably a borohydride, as to readily and quantitatively convert said metal oxide into a conducting metal.

This application is a division of application Ser. No. 441,153, filedNov. 12, 1982, now U.S. Pat. No. 4,517,227.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to the forming of electronic printedcircuits of the thick-layer hybrid type, to certain means adapted forcarrying out the forming process and to the printed circuits formedthereby.

2. Description of the Prior Art:

Hybrid microelectronics has evolved in the art in response to twoconcerns: to solve the problems of bulk and to improve the reliabilityof the circuits. As distinct from a monolithic circuit, which isobtained from a semiconducting substrate in which all of the components(active and passive) are simultaneously produced in the course of but asingle process, a hybrid circuit is produced on an insulating substrateand contains only passive elements; if there are any active elements,namely, semiconductors or integrated circuits, same are later added bysoldering. Hybrid circuits are classified as thin-layer circuits andthick-layer circuits. This classification relates to the thickness ofthe deposited layers: from 0.02 to 10 μm in the case of thin layers, andfrom 10 to 50 μm in the case of thick layers.

For the thin layers, the layers are typically deposited by utilizing thetechniques of vacuum evaporation or cathodic sputtering. For thethick-layer hybrids, the technology used generally consists of printingthe desired circuits directly onto the insulating substrate by means ofconventional silk-screen processes.

Silk-screen printing consists of forcing the ink which it is desired todeposit onto the substrate through the fine meshes of a screen, some ofwhich are blocked by a special lacquer, the free meshes being a veryprecise representation of the half-tone drawing of the circuit to bereproduced.

Any hybrid circuit must be produced on a substrate. The final quality ofthe circuit will depend upon the selection of a good substrate. Thesubstrate generally serves a triple purpose: that of mechanical support,not only for the deposited circuit but also for the active componentswhich it is to receive; that of electrical insulator, its resistivity,therefore, having to be as high as possible and its loss factor as lowas possible; and that of heat dissipator, which therefore requires anexcellent thermal conductivity and a high specific heat. Among thesuitable subtrates, representative are: inorganic materials, such as,for example, ceramics like beryllium oxide, alumina and Pyrex glass; andorganic materials, such as, for example, reinforced resins made fromthermosetting polymeric substances.

Once it has received a conducting, insulating or resistant print in theform of an ink having a well-defined rheology, the substrate is baked orcured in an oven such that the deposit becomes perfectly integral withthe substrate. Finally, the output wires are attached and, afterconnection of the active components, if appropriate, the finished modulecan be coated by encapsulation in a glass or a polymeric resin in orderto protect it from the action of moisture and to make it easier tohandle.

Virtually all thick-layer circuits each receive several successiveprints. In a typical process, the conducting layer is first printed andbaked; the resistors, which will be adjusted to the required values, arethen printed and baked. This process can also include the printing of aninsulating layer. It is thus possible to successively deposit, forexample, conducting insulating, conducting and resistant layers.Briefly, all of the stages of such process can be summarized in thefollowing scheme: ##STR1##

As regards the silk-screen printing inks required for the manufacture ofthe thick-layer circuits immediately above-described, these aretherefore of three types: conducting, resistant and insulating inks. Theconducting and resistant inks typically consist of the combination of ametal extender with a binder and, if appropriate, a diluent making itpossible to adjust the rheological properties of the inks. By modifyingthe chemical structure of the binder, the baking fixes the metal depositto the substrate and makes it possible to obtain the desired finalconducting or resistant circuit. The insulating inks differ from theaforesaid inks essentially by the fact that they do not containconducting metal extenders.

In hybrid circuits, the ink binder commonly consists of inorganicparticles, such as, for example, fusible special glasses in powder form.As these glass-containing inks are baked at about 700° C. to 1,100° C.,their use means that ceramics are preferably selected as the insulatingsubstrates, and powders of non-oxidizable noble metals, in particularplatinum, gold, silver or palladium/gold, palladium/silver orplatinum/gold alloys, or powders of conducting oxides derived from noblemetals, are selected as the conducting metal extenders. Because of theuse of noble metals, the adoption of this technique results in highexpenditure, which will constitute an obstacle to its development. Ittoo has been proposed to use powders of the non-noble metals, such as,for example, copper, but in that case the final baking should be carriedout in a non-oxidizing atmosphere, and this measure complicates thetechnology of the ovens which can be used.

It is possible to replace the aforesaid inorganic binder by an organicbinder, such as, for example, an epoxy resin. The ink-bakingtemperatures required in this case are considerably lower than in thecase of the inorganic binders; same typically range from 100° C. to 300°C. It is then possible to use types of insulating substrates other thanceramics, such as, for example, reinforced organic resins, and to useinks based upon a non-noble metal extender, there being greatly reducedrisks of oxidation of the metal extender during baking carried out, inthe conventional manner, in the atmosphere at from 100° C. to 300° C.However, the inks prepared and baked in this manner entail a unique andsignificant disadvantage: they are difficult to solder. Apart from theirfunction of conducting an electric current, the inks must in fact becontact points for receiving wires or, if appropriate, lugs of activecomponents, which assumes an excellent suitability for soldering, inparticular with tin-based alloys.

In the metallization of insulating substrates, to produce high-qualityconducting printed circuits capable of withstanding soldering withoutdifficulty, it has been proposed to use inks comprising an organicbinder made of a polymeric substance and a metal extender based oncuprous oxide (compare U.S. Pat. Nos. 3,226,256 and 3,347,724). Afterbaking, the deposit obtained is reduced by treatment with an acid, inorder to convert the cuprous oxide to copper metal; this layer of copperis then reinforced by deposition of a ductile metal, which can also becopper, employing a chemical oxidation/reduction reaction. It is knownthat the reduction of cuprous oxide with an acid involves the formationof an unstable cuprous salt which disproportionates to give, on the onehand, cupric salts, and, on the other hand, copper metal, whichdeposits, according to the equation:

    Cu.sub.2 O+2H.sup.+ →Cu.sup.++ +Cu+H.sub.2 O

In a reduction of this type, the yield of copper metal is well below50%: in fact only half of the initial copper is capable of beingconverted to conducting metal and, furthermore, a portion of thisreduced copper is dissolved by the acid agent used. As a result, themetal deposit obtained will only be very slightly conducting. It is forthis reason that, in this technique, recharging of the deposited circuitwith metal is always carried out after the reduction step. The fact thatsuch recharging is carried out by a chemical oxidation/reduction methodand not by an electrolytic method (which does not work) is confirmationof the poorly conducting nature of the original deposit. A firstdisadvantage of this method is the fact that the recharging with metalis a lengthy operation, the deposition rate in fact being on the orderof 1 μm per hour. A second disadvantage is the fact that there will bean increase in the thickness of the original deposit, which may reach100%, and this increase in the thickness is likely to limit the numberof layers deposited by silk-screen printing, in order to avoid anexcessively great loss of inherent flatness of the substrate as a resultof the successive depositions.

Thus, it has proved necessary to develop, in the manufacture of hybridcircuits, a technique by means of which conducting circuits could beeconomically deposited and in a technically simple manner, withoutelectrochemical recharging, the said conducting circuits being suitablefor soldering and not containing extra thicknesses.

Furthermore, as has been explained above, the thick-layer hybridcircuits each receive several successive silk-screen prints. Forexample, a condenser will be formed by the deposition and baking of thelower electrode 1, the printing and baking of the dielectric 2 and theprinting of the upper electrode 3, followed by baking of the assembly(compare FIG. 1 of the drawings, which represents a condenser insection). The manufacturing process therefore involves three successivedepositions with two different inks: a conducting ink and an insulatingink. For other types of hybrid circuits, the manufacturing process mayalso involve additional depositions with a resistant ink. Here again, itproved necessary to develop a technique making it possible to simplifythe procedure for printing the layers, in particular by limiting thenumber of inks required.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofan improved process for the forming of thick-layer hybrid circuits whichotherwise avoids those disadvantages and drawbacks above-outlines, whileat the same time satisfying the aforesaid stated objectives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of typical condenser, including upperand lower electrodes and a dielectric interlayer;

FIG. 2 is a cross-sectional view of a condenser manufactured accordingto this invention; and

FIGS. 3 to 7 are cross-sectional views illustrating the forming of ahybrid circuit according to the invention, the several Figures depictingsaid circuit in various successive intermediate and final stages.

DETAILED DESCRIPTION OF THE INVENTION

More particularly according to this invention, featured is a process forthe manufacture of thick-layer hybrid circuits which comprises printingthe desired circuits on an insulating substrate by the deposition of asuitable ink, in predetermined pattern, and then baking it, thesedeposition and baking operations being repeated as many times asrequired, said process being characterized in that an insulating ink isused which comprises a non-conducting oxide derived from a non-noblemetal, and which has the characteristics of being potentially conductingor potentially resistant, according to its composition, and in that theconducting or resistant nature of the ink is developed, after it hasbeen deposited and baked, by subjecting it to the action of a reducingagent which is specially selected, on the one hand, for its ability toconvert the metal oxide rapidly and quantitatively to conducting metal,and, on the other hand, for its ability to easily modify the magnitudeof the reduction in the thickness of the deposit produced.

Even more particularly, the present invention relates to a process forthe manufacture of hybrid circuits in which the metal extender of theink preferably comprises cuprous oxide, and in which the reducing agentpreferably comprises a borohydride.

The present invention also relates to the potentially conducting andresistant inks utilized in carrying out the subject process.

The present invention also relates to the hybrid circuits obtained bycarrying out the present process.

By the term "potentially conducting ink", there is intended an inkwhich, after baking and reduction, will have a surface resistivity of atmost 0.1 Ω/□. By the term "potentially resistant ink", there is intendedan ink which, after baking and reduction, will have a surfaceresistivity varying from 10 to a value of 10³ Ω/□ or more. Theresistivity sought to be obtained will depend, on one hand, on thecomposition of the ink, and, on the other hand, on the magnitude of thereduction in the thickness of the deposit produced.

In general, the inks intended for carrying out the process according tothe invention consist, before baking, of the combination of at least oneextender based on cuprous oxide with a binder and a diluent enablingadjustment of the rheology of the ink. In the case of the potentiallyconducting ink, it is distinguished by the fact that it contains asingle extender based on cuprous oxide, which represents 40 to 70% ofthe weight of the ink, and a binder which represents 4 to 10% of theweight of the ink. In the case of the potentially resistant ink, it isdistinguished by the fact that it contains a mixture of extenderscomprising 40 to 60% by weight of cuprous oxide and 60 to 40% by weightof an insulating extender which is totally inert under the reductionconditions, such as, for example, powdered glass, alkali metalsilicates, alumina or natural carbonates and sulfates, this mixture ofextenders representing 30 to 60% of the weight of the ink, and a binderwhich represents 10 to 30% of the weight of the ink.

Although inorganic binders can be used, organic binders areadvantageously used for the inks according to the invention. Suitabletypes of organic binders which are especially representative are: resinsof the phenolic type, such as, for example, condensates of phenol,resorcinol, cresol or xylenol with formaldehyde or furfural; resins ofthe unsaturated polyester type, prepared, for example, by reaction of anunsatuated dicarboxylic anhydride with a polyalkylene glycol; resins ofthe epoxy type, such as, for example, the reaction products ofepichlorohydrin with bisphenol A; and resins of the polyimide type, suchas, for example, those obtained by reaction of an unsaturateddicarboxylic acid N,N'-bis-imide with a primary polyamine and, ifappropriate, a suitable adjuvant.

The resin used is generally in the form of a thermosetting prepolymer(having a softening point and still being soluble in certain solvents)at the time of preparation of the ink, and it is in the completelycross-linked form (infusible and totally insoluble) in the ink such asobtained after baking. The resin in the form of a prepolymer, usedinitially, can advantageously contain a hardening catalyst.

Preferably, the binder consists of a resin of the polyimide type,obtained by reaction of an unsaturated dicarboxylic acid N,N'-bis-imidewith a primary polyamine, according to, for example, French Pat. No.1,555,564, U.S. Pat. Nos. 3,562,223 and 3,658,764 and U.S. Pat. No. Re.29,316; hereby expressly incorporated by reference. The polyimide resincan also be obtained by reaction of the bis-imide with the polyamine andvarious adjuvants, such as, for example, mono-imides (according toFrench Pat. No. 2,046,498), monomers, other than imides, containing oneor more polymerizable groups of the type CH₂ ═C< (according to FrenchPat. No. 2,094,607), unsaturated polyesters (according to French Pat.No. 2,102,878) or hydroxylic organosilicon compounds (according toFrench Pat. No. 2,422,696); these also hereby being expresslyincorporated by reference. In the case where such adjuvants are used, itwill be appreciated that the polyimide resin in the form of a prepolymercan be: the reaction product of a bis-imide and a polyamine with anadjuvant, or the reaction product of a bis-imide prepolymer and apolyamine with an adjuvant, or alternatively a mixture of a bis-imideprepolymer and a polyamine with an adjuvant.

The polyimide resin in the form of a prepolymer, used initially, canadvantageously contain small amounts of a free-radical polymerizationinitiator, such as, for example, dicumyl peroxide, lauroyl peroxide orazobisisobutyronitrile, or of an anionic polymerization catalyst, suchas, for example, diazobicyclooctane.

In the present invention, it is even more preferred to use a polyimideresin resulting from the reaction of a bis-maleimide, such asN,N'-4,4'-diphenylmethane-bis-maleimide, with a primary diamine, such as4,4'-diaminodiphenylmethane, and, if appropriate, one of the variousadjuvants mentioned above.

As regards the diluents which can be used, they generally consist oforganic liquids having a variety of viscosities, which can range, forexample, from 1 cpo to several tens of poises, the chemical nature ofthe organic liquids not being critical, provided that they are notsolvents for the binder used in the form of a thermosetting prepolymer,or solvents for the lacquer which blocks some of the meshes of a silkscreen. Suitable types of diluents which are particularly representativeare: ethers of alkanols and glycols, such as, for example, diglyme andbutoxyethanol; and terpene alcohols, such as, for example, α-, β-, andγ-terpineols.

A diluent which is very particularly suitable, especially when theselected binder belongs to the preferred group of the resins of thepolyimide type defined above, consists of α-, β- or γ-terpineol.

As regards the composition of the inks, it still remains to be specifiedthat the cuprous oxide, the totally inert extender and the binder areused in the form of powders. The particle size of the extenders isgenerally from 0.1 to 5 μm and the particle size of the binder isgenerally from 5 to 40 μm.

It has been indicated above that the resistivity which it is desired toobtain for the deposit will depend on the magnitude of the reduction inthe thickness of the deposit produced, as well as on the composition ofthe ink. Therefore, the nature of the reducing treatment carried outafter baking will now be discussed in greater detail.

Thus, it has been found that the conversion of the cuprous oxide tocopper metal can be carried out easily and quantitatively by reactionwith borohydrides. The conversion is represented by the equation:

    4Cu.sub.2 O+BH.sub.4.sup.- →8Cu+B(OH).sub.3 +OH.sup.-

The ease with which this reaction takes place is probably due to thecatalytic effect of the copper metal, which could be explained by theintermediate formation of unstable copper hydride.

The borohydrides which can be used in the present invention includesubstituted borohydrides, as well as unsubstituted borohydrides. It ispossible to use substituted borohydrides in which at most three hydrogenatoms of the borohydride ion have been replaced by inert substituents,such as, for example, alkyl radicals, aryl radicals or alkoxy radicals.It is preferred to use alkali metal borohydrides in which the alkalimetal moiety consists of sodium or potassium. Typical examples ofsuitable compounds are: sodium borohydride, potassium borohydride,sodium diethylborohydride, sodium trimethoxyborohydride and potassiumtriphenylborohydride.

The reducing treatment is carried out in a simple manner by contactingthe ink, after baking, with a solution of borohydride in water, or in amixture of water and an inert polar solvent, such as, for example, alower aliphatic alcohol. Purely aqueous solutions of borohydride arepreferred. As regards the concentration of these solutions, it can varyover wide limits and preferably ranges from 0.5 and 10% (by weight ofborohydride in the solution). The reducing treatment can be carried outat elevated temperature, but it is preferred to carry it out at atemperature close to ambient temperature, for example, at from 15° C. to30° C. As regards the course of the reaction, it should be noted that itgives rise to B(OH)₃ and to OH⁻ ions, the effect of which is to cause anincrease in the pH of the medium during the reduction. Now, at high pHvalues, for example, above 13, the reduction is slowed down and it canthus be advantageous to carry out the reaction in a buffered medium suchas to provide a well-defined rate/extent of reduction.

It is mainly by varying the time of treatment that it is possible toeffectively modify the magnitude of the reduction in the thickness ofthe deposit and consequently to modify the value of the resistivitieswhich will develop. The time required for treatment is typically fairlyshort and usually ranges from one minute or less to about ten minutes.For a given treatment time, it is also possible to influence the rate ofreduction by adding a variety of reduction accelerators to the medium,such as, for example, boric acid, oxalic acid, citric acid, tartaricacid or metal chlorides, such as the chlorides of cobalt(II),nickel(II), iron(II), manganese(II) or copper(II). At the beginning ofthe reduction, the reaction essentially involves the cuprous oxideparticles which are located on the surface of the deposit and which arein direct contact with the reducing agent. Because of the catalyticeffect of the copper metal, the reduction reaction will then continuewithin the bulk of the deposit, even though the ink does not have aparticularly marked hydrophilic nature. Depending upon the time oftreatment, the thickness of the ink which is in fact subjected toreduction can represent from 10 to 100% of the thickness of the deposit.

It will be appreciated that the resistivity values are also a functionof the geometry of the conducting or resistant components selected. Thisaspect of the subject forming process thus requires discussion of themethods by which the desired circuits can be constructed on theinsulating substrate.

A first method consists of depositing the inks by techniqueconventionally used in the manufacture of hybrid circuits, namely, thesilk-screen technique. However, according to the present invention, theproduction of a conducting or resistant circuit entails effecting areducing treatment upon the deposit after baking the ink and beforemaking the next print. It must be understood that, in the case of theprinting of an insulating circuit, the ink used, which can be eitherpotentially conducting or potentially resistant, will not, on the otherhand, undergo any reducing treatment after baking.

The ability of the extender based on cuprous oxide to impart, to thesame ink, firstly an excellent insulating nature at the beginning of theoperation and secondly an excellent conducting or resistant nature afterreduction, permits a striking modification of the conventional methodheretofore employed in the art, as above-described. It is now possiblefor the selective deposit obtained by silk-screening printing to bereplaced by a uniform deposit of a potentially conducting or resistantink, for example by immersion, spraying, calendering, knife coating ortransfer, followed, after baking of the ink and in the case whereconducting or resistant circuits are to be produced, by selectivedeveloping of the conducting or resistant parts by reduction using amasking technique; it is this replacement which constitutes the subjectof a second, particularly advantageous embodiment. This maskingtechnique is normally used in the field of printed circuits and, to ourknowledge, has not as yet been applied in the field of thick-layerhybrid circuits. It will be recalled that this technique consists ofcovering the uniformly deposited and baked layer of ink with aphotosensitive resin, also called a photoresist, and in subsequentlyexposing this photosensitive resin, through a mask, to radiation rich inultraviolet rays. The photosensitive resins are organic polymers whichundergo chemical conversion under the influence of ultraviolet rays,modifying their solubility in certain solvents. The purpose of the maskis to permit the radiation to selectively pass therethrough, in thepattern of the circuit to be printed. It frequently comprises a finelayer of chromium deposited onto a glass plate or is comprised of aphotographic gelatin film. If a positive photoresist is used, thenon-opaque portions of the mask correspond to the desired circuit;conversely, in the case of a negative photoresist, it is the opaqueareas which correspond to the desired circuit. Developing is thencarried out; this consists of dissolving the appropriate portions of thephotosensitive resin, namely, according to the nature of thephotoresist: either the portions rendered soluble by exposure, in thecase of a positive photoresist, or the unexposed parts not hardened byexposure, in the case of a negative photoresist. The potentiallyconducting or resistant ink therefore appears at the locationscorresponding to the dissolved portions. The remaining operationscomprise reductive attack of the ink exposed in this manner, followed byremoval of the layer of photosensitive resin which remains and which hasconsequently become superfluous; as the ink which has been protected bythe photoresist is not subjected to any reductive attack, it remainsinsulating.

The utilization of this second method is therefore very advantageous,since it is now possible, beginning with a single potentially conductingink, to select conducting zones and insulating zones on one and the samelayer of deposit. In the same fashion, beginning with a potentiallyresistant ink, it is also possible to select resistant zones andinsulating zones on one and the same layer of deposit; moreover, for agiven ink, the resistant zones do not have a single resistivity valuebut variable resistivity values, which can easily be adjusted by virtueof the action of the reducing treatment.

Again considering the case of a condenser, it can now be manufactured inthe following simplified manner: firstly, a first layer 4 of apotentially conducting ink is deposited in a non-selective manner; then,after baking, the reducing treatment is applied in the presence of themask, which makes it possible to selectively develop the lower electrode4a and a dielectric zone 4b; then, a second layer 5 of the same ink isdeposited in a non-selective manner, and the selective reduction of thisink makes it possible to form the upper electrode 5a and anotherdielectric zone 5b (compare FIG. 2 of the Figures of Drawing, whichillustrates this condenser in cross-section). Compared with the priorstate of the art of using silk-screen printing, not only is the requirednumber of deposits reduced from three to two, but also the requirednumber of inks is reduced from two (conducting and insulating inks) toone (potentially conducting ink). In the same manner, the techniqueprovided by the present invention makes it possible for all three of theinks heretofore employed in the field of hybrid circuits (conducting,resistant and insulating inks) to be replaced by only two inks, the onebeing potentially conducting and the other potentially resistant.

A third embodiment of forming the desired circuits which canadvantageously be carried out consists of using both the silk-screentechnique and the masking technique. In this embodiment, it is possibleto use the silk-screen technique to produce the first level (or levels)of circuits and then to use the masking technique to produce the nextlevel (or levels) of circuits. It is also possible to carry out thesequence comprising one or more depositions by the silk-screentechnique, then one or more depositions by the masking technique andthen one or more depositions by the silk-screen technique. The procedurewhich consists of producing uniform deposits by the masking techniqueafter deposits by the silk-screen technique permits a levelling of theentirety of the substrate, thus making it possible to undertake theproduction of subsequent deposits under good conditions of inherentflatness. The conventional silk-screen technique in fact gives rise,after deposition of the ink, to relief zones which correspond to thepaths of the desired circuit, and to recessed zones which correspond tothe uncovered portions of the substrate, and it will be readily beunderstood that, when successive deposits are produced, this lack ofinherent flatness of the substrate will increase substantially. Becauseof a uniform initial deposit, the masking technique, on the other hand,will result in perfectly planar layers, even after reduction, thereforemaking it possible, when it is carried out after deposits have beenapplied by the silk-screen technique, to easily correct or adjust thelack of inherent flatness generated by silk-screen printing.

The methods of deposition immediately above-described result inpotentially conducting or resistant layers having a thickness, afterbaking, which typically ranges from 10 to 50 μm and more usually rangesfrom 15 to 35 μm.

Generally, once deposited, the inks are dried before being subjected tobaking. This drying, at temperatures on the order of 50° C. to 100° C.,can be carried out, in particular, in an oven, on a heating plate or byinfra-red radiation. The drying time is on the order of one minute toabout ten minutes. After drying, the inks are baked at a highertemperature in order to complete the removal of the diluent and to bindthe deposit to the substrate and cause them to acquire their definitivecharacteristics. The baking temperatures generally range from 100° C. to300° C. In the event that the ink binder belongs to the preferred groupof resins of the polyimide type, the baking temperature preferablyranges from 140° C. to 200° C. The baking is generally carried out inambient air, in a static oven or a tunnel oven.

As regards the remainder of the subject process, the insulatingsubstrates can be fabricated from any one of the materials known forsuch purpose in the prior art. Preferred are the reinforced polymericresins in which the polymeric matrix comprises those resins noted abovewith reference to the binder for the inks according to the presentinvention. Reinforced resins of the phenolic type, of the unsaturatedpolyester type, of the epoxy type, or of the polyimide type, aretherefore eminently suitable. These substrates are in general completelypolymerized laminates in which one or more of the following principalreinforcing means are present: cellulosic papers, cotton fabrics,asbestos fabrics and papers, glass fabrics and glass mats. Other typesof reinforcing fillers can of course also be used. Laminates of thistype are well known in the art and thus will not be more fullycharacterized.

It will be appreciated that the technique immediately above-describedenables obtainment of conducting paths which are of course perfectlysuited for soldering.

It is self-evident that many modifications can be made to theembodiments which have just been described, in particular, by thesubstitution of equivalent means, for example, as regards the non-noblemetal oxide which can be used, without thereby departing from the scopeof the present invention. Thus, the cuprous oxide can be replaced byother non-noble metal oxides in which the state of oxidation is selectedsuch as to permit easy reduction of the oxide by the borohydride;exemplary of suitable such oxides are nickel(II) oxide, cobalt(II)oxide, lead(II) oxide, cadmium(II) oxide, chromium(III) oxide,antimony(III) oxide and tin(IV) oxide.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLE 1

In this example is described a "potentially conducting ink" according tothe present invention.

The following ingredients were dry mixed:

(i) 40.5 g of cuprous oxide having a particle size ranging from 0.5 to 2μm; and

(ii) 4.5 g of a powder, having a particle size of less than 30 μm, of aprepolymer prepared from N,N'-4,4'-diphenylmethane-bis-maleimide and4,4'-diaminodiphenylmethane (molar ratio bis-imide/diamine--2.5), havinga softening point of 105° C.

30 g of α-terpineol, containing 0.09 g of dicumyl peroxide dissolvedtherein, were then incorporated into the pulverulent mixture obtained asabove. The entire mass was homogenized in a mortar. In the case wherethe various compounds mentioned above were used in larger amounts, thehomogenization was carried out, for example, in crushing rolls or inother types of apparatus, such as those used for homogenizing paints.The rheology of the paste prepared in this manner was perfectly suitedfor use, in particular, in silk-screen printing.

EXAMPLE 2

In this example is described a "potentially resistant ink" according tothe present invention.

This ink was prepared in the manner indicated in Example 1, but, on theone hand, the 40.5 g of cuprous oxide were now replaced by a mixture of15 g of cuprous oxide with 15 g of powdered glass, having a particlesize of less than 2 μm, and, on the other hand, the 4.5 g of polyimideprepolymer were now replaced by 15 g of the same prepolymer. Therheology of the paste prepared in this manner was again perfectly suitedfor use, in particular, in silk-screen printing.

EXAMPLE 3

In this example is described the production of a conductive circuit bythe silk-screen technique of selective deposition.

The insulating substrate employed was a 12-ply laminate comprising 35%by weight of FR4-type epoxy resin and 65% by weight of glass fabric.

The potentially conducting ink prepared in Example 1 was deposited ontothis substrate through a 325 mesh stainless steel silk screen whichfaithfully reproduced the pattern of the circuit sought to be formed.The resulting deposit and also the resulting definition of the patternwere of good quality.

The substrate was then dried for about ten minutes in a ventilated ovenat 60° C., and then baked for 1 hour at 175° C. After the drying andbaking steps, the thickness of the resultant deposit was 18 μm.

After cooling, this red-colored deposit was then immersed for one minutein an aqueous solution of sodium borohydride having a concentration of1%. The reducing solution was at ambient temperature, namely, at about20° C., and was stirred. After a treatment time of one minute, theinitially red deposit became red-brown and, after rinsing and drying, ithad a resistivity of 0.05 Ω/□.

EXAMPLE 4

In this example is described the production of a conductive circuit bythe technique of uniform deposition, followed by selective reductionusing a mask.

In this example, the insulating substrate employed was a 12-ply laminatecomprising 35% by weight of polyimide resin prepared fromN,N'-4,4'-diphenylmethane-bis-maleimide and 4,4'-diaminodiphenylmethane(molar ratio bis-imide/diamine=2.5), and 65% by weight of glass fabric.

The potentially conducting ink prepared in Example 1 was depositeduniformly over the entire surface of the substrate by knife coating.After drying and baking for 1 hour at 160° C., the resulting deposit hada thickness of 27 μm.

A negative photoresist, comprising the combination of a resin based oncyclized polyisoprene and of paraazidobenzacyclohexanone as asensitizer, was then deposited uniformly over the entire surface of theink by spraying. After drying, the layer of photoresist was imagewiseexposed through a mask (made of photographic gelatin and reproducing thenegative pattern of the circuit sought to be formed) under the light ofa mercury vapor lamp for 20 seconds, and the image was then developed bydipping the substrate in pure xylene for 1 minute, such as to remove theportions of the photoresist which had not been exposed and whichtherefore became soluble in media which did not affect the exposedportions. The zones which are to be conductive thus appeared at thelocations where dissolution was effected.

Following the procedure indicated in Example 3, the potentiallyconducting zones of the substrate which were developed in this mannerwere then subjected to the action of the reducing solution of sodiumborohydride. Finally, the remaining photoresist was totally removed andthe zones then appearing were insulating; this operation consisted ofdissolving the photoresist by attack (stripping) with a suitable productmarketed by Rhone-Poulenc under the trademark RP 1 332, the processbeing carried out at 90° C.-100° C.

A perfectly planar, thick-layer conducting circuit was ultimatelyobtained, which was capable of receiving further deposits. Itsresistivity was 0.065 Ω/□.

EXAMPLE 5

In this example it is illustrated that the conducting circuits obtainedaccording to the invention are perfectly suitable for soldering.

After reduction, rinsing and drying, the conducting circuit produced inExamples 3 or 4 was immediately passed between a flexible pressureroller, on the one hand, and a roller drawing a film of tin-basedeutectic, on the other hand. It was found that the conducting zones werethen covered with a continuous layer of a copper/tin alloy, whichevidenced the formation of a complete coating of solder.

EXAMPLE 6

In this example is described the use of a potentially resistant ink.

The use of the potentially resistant ink according to Example 2,following the procedures described in Examples 3 (silk-screen technique)and 4 (masking technique), resulted in the production of resistantlayers which displayed resistivities of 227 Ω/□ and 325 Ω/□,respectively.

EXAMPLE 7

In this example is described the production of a hybrid circuit having aplurality of thick layers by utilizing both the silk-screen techniqueand the masking technique.

The successive operations which were carried out on the substrate areshown in FIGS. 3 to 7 of the Figures of Drawing, depictingcross-sections of the substrates obtained after each operation.

The insulating substrate employed was the 12-ply laminate described inExample 3. The following operations were carried out:

(i) the potentially conductive ink according to Example 1 was depositedselectively onto the substrate by silk-screen printing, and the inkdeposit was then dried, baked and reduced in the manner indicated abovein Example 3. The conducting zones which thus appeared, and whichconstituted the first level of conductors, are represented by referencenumeral 6 in FIG. 3;

(ii) secondly, the same ink as above was deposited uniformly over theentire surface of the substrate resulting from step (i). After dryingand baking, a perfectly planar, insulating layer was obtained,represented by the reference numeral 7 in FIG. 4;

(iii) thirdly, using masking technique, the insulating layer 7 wasreduced selectively at the location where it was desired to establish aninterlayer conducting junction. The conducting zone which was developedin this manner is represented by reference numeral 8 in FIG. 5;

(iv) fourthly, the same potentially conducting ink was again deposited,uniformly, as in step (ii), onto the substrate resulting from step(iii). The insulating layer obtained after drying and baking isrepresented by reference numeral 9 in FIG. 6; and

(v) fifthly, by again utilizing the masking technique, the insulatinglayer 9 was then reduced selectively according to the pattern selectedfor this second level of conductors. The conducting zones which thenappeared are represented by reference numeral 10 in FIG. 7.

The operations (ii), (iii) and (iv) are repeated as many times asrequired to provide the desired number of additional levels ofconductors. The operations (iv) and (v) are repeated as many times asrequired to produce the desired number of additional conductingcircuits. By using potentially resistant inks, it is possible tointercalate resistant circuits of low thermal dissipation between thelayers, it being necessary for the resistant circuits of high thermaldissipation to be brought to the surface.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims.

What is claimed is:
 1. An electrically insulating, potentiallyconductive ink composition adapted for deposition fabrication ofthick-layer hybrid electronic printed circuits, comprising from about40% to about 70% of a particulate cuprous oxide extender, from about 4%to about 10% of a particulate binder, and a diluent therefor, whichbinder is curable in a temperature range of from approximately 100° to300° C., wherein the ink composition is insulating when baked andconductive when suitably reduced.
 2. An electrically insulating,potentially resistant ink composition adapted for deposition fabricationof thick-layer hybrid electronic printed circuits, comprising from about30% to about 60% of a particulate extender admixture of from about 40%to about 60% particulate cuprous oxide and about 60% to about 40%particulate inert insulating extender, from about 10% to about 30% of aparticulate binder, and a diluent therefor, which binder is curable in atemperature range of from approximately 100° to 300° C., wherein the inkcomposition is insulating when baked and is resistant if suitablyreduced.
 3. The ink composition of claim 2, wherein said insulatingextender is comprised of powdered glass, an alkali metal silicate, analumina, a carbonate or a sulfate.
 4. The ink composition of claims 1 or2, wherein said binder is comprised of a thermosetting prepolymer. 5.The ink composition of claim 4, wherein the particle size of saidextender(s) is in the range of from about 0.1 to about 5.0 microns andthe particle size of said binder is in the range of from about 5 toabout 40 microns.
 6. The ink composition of claim 4, wherein saidprepolymer is comprised of a polyimide resin reaction product of anunsaturated dicarboxylic acid N,N'-bis-imide with a primary polyamine.7. The ink composition of claim 6, further comprising an adjuvant of oneof a mono-imide, an ethylenically unsaturated polymerizable comonomer,an unsaturated polyester or an hydroxylated organosilicon compound. 8.The ink composition of claim 4, wherein said diluent is comprised of anether of an alkanol or glycol, or a terpene alcohol.